Implantable medical device, medical system and method for data communication

ABSTRACT

An implantable medical device including a data communication device that includes a device to alter and/or generate an oscillatory electric field imposed on body tissue surrounding the implantable medical device when the implantable medical device is in its implanted state. The device that alters an oscillatory electric field modulates an impedance of body tissue surrounding the implantable medical device when the implantable medical device is in its implanted state and within an oscillatory electric field. The device that alters an oscillatory electric field includes a device that generates an oscillatory electric field that is phase-synchronized with an oscillatory electric field imposed on body tissue surrounding the implantable medical device when the implantable medical device is in its implanted state.

This application is a divisional application of U.S. patent applicationSer. No. 14/157,389, filed on 16 Jan. 2014, which claims the benefit ofU.S. Provisional Patent Application 61/761,707, filed on 7 Feb. 2013,the specifications of which are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

At least one embodiment of the invention generally relates toimplantable medical devices and data communication from implantablemedical devices to an external device.

2. Description of the Related Art

Typically, implantable devices, in particular implantable medicaldevices, such as implantable therapy and/or monitoring devices includingpacemakers, cardioverters and defibrillators or the like, may includedata communication means to transmit data from the implantable medicaldevice to an external device or vice versa.

A system for data communication with a medical device thus, generally,includes an implantable medical device and an external device such as aprogrammer.

A typical implantable medical device comprises a battery, a monitoringand/or therapy control unit, in some cases additionally one or moretherapy units such as stimulation units and a memory for storing controlprogram and/or data sensed by the implantable medical device. If theimplantable medical device is a pacemaker or an implantablecardioverter/defibrillator (ICD), generally, the therapy units comprisestimulation units for generating and delivering electric stimulationpulses to a patient's heart tissue (myocardium).

Generally, in order to transmit data sensed by the implantable medicaldevice to an external device, a telemetry unit may be provided.Typically the telemetry unit may be configured to allow bidirectionaldata communication, that is, the telemetry unit may transmit and receivedata wirelessly.

Limited battery capacity of an implantable medical device generallycalls for energy-efficient data communication. An implantable medicaldevice with limited battery power typically requires a low powercommunication scheme in order to program it and to download acquireddata. With extremely low power communication, generally, more data maybe transmitted more often.

Typical communication schemes used for data communication by a telemetryunit involve either RF or magnetic communication. Generally, RFfrequencies of ˜400 or ˜900 MHz or magnetic coupling in the 100s of kHzrange require several mA of current to transmit and receive data. Suchhigh current requirements are typically out of reach of devices withbattery capacities of at most a few hundred mAh.

In addition, RF schemes generally require large antennas and magneticcoupling generally requires large transmit and receive coils forcommunication. The space available in miniaturized implants, typically,would not allow such large coils or antennas.

In view of the above, there is a need for a low power communicationscheme that does not employ RF or magnetic coupling.

BRIEF SUMMARY OF THE INVENTION

Objectives of the invention according to at least one embodiment of theinvention include providing an alternative implantable device and analternative data communication method that result in little batterydrain from the medical device's battery.

According to one or more embodiments of the invention, an implantablemedical device may be provided that includes one or more datacommunication devices which may include one or more devices that mayalter an oscillatory electric field imposed on body tissue surroundingthe implantable medical device when the implantable medical device is inits implanted state. In embodiments of the invention, the one or moredata communications devices may include one or more devices that maygenerate a second oscillatory electric field, which may be synchronizedto a first oscillatory electric field imposed on body tissue surroundingthe implantable medical device.

In at least one embodiment, the one or more devices that may alter anoscillatory electric field may modulate an impedance of a volume of bodytissue surrounding the implantable medical device when the implantablemedical device is in its implanted state and within an oscillatoryelectric field.

In embodiments of the invention, the one or more devices that may alteran oscillatory electric field may include one or more devices that maygenerate an oscillatory electric field that is phase-synchronized withan oscillatory electric field imposed on body tissue surrounding theimplantable medical device when the implantable medical device is in itsimplanted state.

In at least one embodiment, the one or more devices that may generate asecond oscillatory electric field may include one or more devices thatmay sense the phase of a first oscillatory electric field by theimplantable medical device, and one or more devices that may generatethe second oscillatory electric field in a manner that is phasesynchronized to the sensed first electric field while the medical deviceis in its implanted state.

One or more embodiments of the invention may use modulation of impedancewithin an electric field to communicate between a small medical deviceimplanted in the heart or body and an external programmer/communicationdevice, which may also be implanted in the body. Alternatively,embodiments of the invention may generate a small electric field tofacilitate this same communication, thus modulating the electric fieldimparted on the implantable medical device. In one or more embodiments,the external device may usecutaneous or implanted electrodes to impartan oscillatory electric field on the body that encompasses the medicaldevice.

By way of one or more embodiments, during impedance modulation, themedical device, within the field, may alternate shorting and opening ofan internal connection between two electrodes on its surface to changethe impedance across the space of the device. In at least oneembodiment, the changes in impedance may be sensed by the externaldevice as changes in either current or voltage. The changes in voltageor current may be modulated to form a communication scheme to transmitdata. In one or more embodiments, the external device's electric fieldmay be additionally used to transmit data to the implanted medicaldevice.

According to at least one embodiment of the invention, during electricfield transmission, the external device may use cutaneous or implantedelectrodes to impart an oscillatory electric field on the body thatencompasses the implanted medical device. The implanted device, in oneor more embodiments, may then monitor this field and may generate asmall electric field that may be phase-synchronized with the externaldevice's field in order to facilitate reception of this small field bythe external device using a lock-in (phase-locked) amplifier. Theexternal device's electric field may not be continuously generated, andmay be additionally used to transmit data to the implanted medicaldevice. In one or more embodiments, the small changes in the body'selectric field may be sensed by the external device as changes involtage or current and may be demodulated to form a communicationsscheme from the implanted device to the external device.

Embodiments of the invention may allow an implantable medical devicewith limited battery supply the ability to transmit increased amount ofdata while using very little power. Data transmission, in at least oneembodiment, may be achieved by modulating an electric field imparted onthe body by altering the impedance of the transmission medium and readby the receiver as changing voltage or current. In embodiments of theinvention, continuous medium rate data transmission may be achievedwhile using very little battery power.

Preferably, in at least one embodiment, the implantable medical deviceduring impedance modulation may further include at least one switchingdevice and at least two electrodes that may be connected to the at leastone switching device, such that the at least two electrodes may beelectrically connected or disconnected, respectively, by the at leastone switching device. In at least one embodiment, the at least twoelectrodes may cause a change of impedance when the implantable medicaldevice is in its implanted state and the electrodes may be connected ordisconnected, respectively, using the at least one switching device.

The at least one switching device, in at least one embodiment, may beconnected to a switch control that may sense an oscillatory electricfield imposed on body tissue surrounding the implantable medical devicewhen the implantable medical device is in its implanted state. Thus, inone or more embodiments, it is possible to synchronize connecting anddisconnecting of the at least two implantable electrodes with theoscillatory electric field imposed on the body tissue surrounding orencompassing the implantable medical device.

To implement such synchronizing, in at least one embodiment of theinvention, the switch control may include a phase-locked loop (PLL) anda frequency divider, wherein the phase-locked loop may lock in afrequency of an oscillatory electric field imposed on body tissuesurrounding the implantable medical device when the implantable medicaldevice is in its implanted state. The frequency divider, in embodimentsof the invention, may be connected to the phase-locked loop and maydivide a frequency signal outputted by the phase-locked loop. Thus, inat least one embodiment, the implantable medical device may generate acode that may represent data to be transmitted from the implantablemedical device to an external device, wherein the clock for such codemay be a fraction of the frequency of the oscillatory electric fieldimposed on the body tissue surrounding the implantable medical device.According to one or more embodiments, if the frequency of the dividedsignal is sufficiently low, on the order of 1:100, the need for the PLLwould be eliminated. In this case the maximum decrease in efficiency dueto lack of phase lock with the external electric field may be onepercent, or one out of 100 cycles of the imparted field.

According to at least one embodiment, the switch control may beconnected to the at least two electrodes and may sense an oscillatoryelectric field imposed on body tissue via the at least two electrodes.The switch control, in embodiments of the invention, may include aband-pass filter, wherein the band-pass filter may filter a signal fedto the phase-locked loop.

In at least one embodiment, the at least one device that may alter anoscillatory electric field may include at least one field generatingdevice that may generate an oscillatory electric field that may bephase-synchronized with an oscillatory electric field imposed on bodytissue surrounding the implantable medical device when the implantablemedical device is in its implanted state. An implantable medical deviceaccording to at least one embodiment may further include at least onefield generation control device that may be operatively connected to theat least one field generating device and may control the at least onefield generating device in response to an oscillatory electric fieldimposed on body tissue surrounding the implantable medical device whenthe implantable medical device is in its implanted state.

Objectives of the invention may be further achieved by a datacommunication system including an implantable medical device asdescribed above and an external device that may include or may beconnected to at least two cutaneous or implanted electrodes. In at leastone embodiment, the external device may include at least one externalfield generating device that may generate an oscillatory electric fieldthat may be imposed across the implanted medical device via the at leasttwo cutaneous or implanted electrodes. The external device, in one ormore embodiments, may include at least one sensing device that may sensealterations of body impedance and/or an oscillatory electric fieldgenerated by the implantable medical device when the implantable medicaldevice is in its implanted state.

The external device, in one or more embodiments, may include a lock-inamplifier, an AM demodulator that may demodulate the amplitude-modulatedsignals and an analog-to-digital converter, wherein theanalog-to-digital converter may be operatively connected to the AMdemodulator and the lock-in amplifier, and wherein the analog-to-digitalconverter may produce a signal that may represent a signal transmittedby the implantable medical device.

Objectives of the invention according to at least one embodiment may befurther achieved by a method of communicating data from an implantablemedical device to an external device, wherein the method may include:

-   -   imposing an oscillatory electric field in body tissue        encompassing an implantable medical device,    -   sensing the imposed oscillatory electric field using the        implantable medical device,    -   altering the oscillatory electric field using the implantable        medical device by inducing an alternating change of impedance        using the implantable medical device or by generating a small        oscillatory electric field using the implantable medical        device,—and/or    -   generating an oscillatory electric field that may be phase        synchronized to the imposed electric field using the implantable        device, and sensing the change of impedance or the small        oscillatory electric field generated by the implantable medical        device, respectively, using an external device having or being        connected to at least two cutaneous electrodes.

Preferably, in at least one embodiment, the step of altering theoscillatory electric field using the implantable medical device may beperformed using at least two electrodes that may be operativelyconnected to or be part of the implantable medical device, and at leastone switch that may be operatively connected to the at least twoelectrodes. In embodiments of the invention, the at least one switchingdevice may connect and disconnect the at least two electrodes in analternating manner according to a code-representing data that may betransmitted from the implantable medical device to the external device.In one or more embodiments, the alternating connecting and disconnectingmay cause a detectable change of impedance for the imposed oscillatoryelectric field.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of at least oneembodiment of the invention will be more apparent from the followingmore particular description thereof, presented in conjunction with thefollowing drawings wherein:

FIG. 1 shows a representation of a communication system, including animplantable medical device in its implanted state and an externaldevice, according to one or more embodiments of the invention.

FIG. 2 shows a more abstract representation of the system depicted inFIG. 1 according to one or more embodiments of the invention.

FIG. 3 shows a more detailed representation of an implantable deviceaccording to one or more embodiments of the invention.

FIG. 4 shows a more detailed representation of an external deviceaccording to one or more embodiments of the invention.

FIG. 5 shows a circuit diagram for a simulation scenario for simulatingimpedance change based data communication according to one or moreembodiments of the invention.

FIG. 6 shows a plot illustrating a simulation result achieved by usingthe diagram depicted in FIG. 5 according to one or more embodiments ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out at least one embodiment of the invention. This descriptionis not to be taken in a limiting sense, but is made merely for thepurpose of describing the general principles of the invention. The scopeof the invention should be determined with reference to the claims.

FIG. 1 shows a representation of a communication system, comprisingincluding an implantable medical device in its implanted state and anexternal device, according to one or more embodiments of the invention.As shown, one or more embodiments of the invention, may utilize animplanted pacemaker such as an implantable medical device 10, aprogrammer/communication device such as external device 12, andcutaneous electrodes 14 that may be placed on either side of the heart.In at least one embodiment, the external device 12 may induce anoscillating electric field between the electrodes at 100 kHz-1 MHz, forexample, at a known voltage or current. The medical device implanted inthe heart may be between the two electrodes in embodiments of theinvention.

FIG. 2 shows a more abstract representation of the system depicted inFIG. 1 according to one or more embodiments of the invention.

FIG. 3 shows a more detailed representation of an implantable device 10according to one or more embodiments of the invention. It should benoted that an implantable medical device, according to embodiments ofthe invention, such as an implantable heart stimulator may include abattery, a monitoring and/or therapy control unit, one or more therapyunits such as stimulation units and a memory that may store controlprogram and/or data sensed by the implantable medical device. If theimplantable medical device is a pacemaker or an implantablecardioverter/defibrillator (ICD), in at least one embodiment, thetherapy units may include stimulation units that may generate anddeliver electric stimulation pulses to a patient's heart tissue(myocardium). Stimulation units may be connected to stimulationelectrode leads in embodiments of the invention.

According to one or more embodiments, in order to transmit data sensedby the implantable medical device 10 to an external device 12, atelemetry unit may be provided. In at least one embodiment, thetelemetry unit may allow a bidirectional data communication, that is,the telemetry unit may transmit and receive data wirelessly. FIGS. 3 and4 illustrate details of internal and external therapy units,respectively, according to one or more embodiments the invention.

Referring to FIG. 3, in at least one embodiment, the implantable medicaldevice 10 may include two (implantable) electrodes 18 that may contactbody tissue surrounding the implantable medical device 10 in itsimplanted state. In embodiments of the invention, at least one switchmay be at least one simple switch 16 that may connect and/or disconnect,respectively, the two electrodes 18 in order to establish or interruptan electrical connection between the electrodes. The electricalconnection across the at least one switch 16 may be of low ohmicresistance, sufficiently low to detectably decrease the volume impedanceof the tissue surrounding the implanted medical device. In at least oneembodiment, the at least two electrodes 18 of the implantable medicaldevice 10 may be arranged on the external surface of a hermeticallysealed housing encapsulating the implantable medical device 10.According to one or more embodiments, parts of the housing itself mayform the at least two electrodes 18.

In order to control the at least one switch 16, by way of one or moreembodiments, the implantable medical device 10 may include a frequencydivider 20 that may be connected to a sine-to-square convertingcomparator 22 that in turn may be connected to a phase-locked loop 24.Phase-locked loop 24, in embodiments of the invention, may be connectedto the electrodes 18 via a band pass filter 26. Phase-locked loop 24 andfrequency divider 20 may be part of a switch control of implantablemedical device 10.

In at least one embodiment of the invention, the field induced betweenthe cutaneous electrodes may be sensed by the implantable medical device10. The implantable medical device 10 may lock in the frequency of theelectric field using a phase-locked loop 24. Once the implantablemedical device 10 is locked on to the frequency of the external device'sinduced field, in one or more embodiments, it may activate the at leastone switch 16 between two electrodes 18 that may be in the field (asshown in FIGS. 2 and 3 for example) in synch with the frequency of theelectric field.

In more detail, according to one or more embodiments, the implantablemedical device 10 may sense the imposed oscillatory electric signal asinput signal that may be detected via the electrodes 18 or across aresistor. Thus, in at least one embodiment, the implantable medicaldevice may include an input sine signal that may be detected as analternating voltage across electrodes 18 or across a resistor. Thisinput sine signal may be band pass filtered by band pass filter 26. Arepresentation of the band pass filtered signal is shown in diagram (a)of FIG. 3.

By way of one or more embodiments, the band pass filtered input sinesignal may be fed to the phase-locked loop (PLL) 24 that may lock in thefrequency of the input sine signal. Phase-locked loop 24, in at leastone embodiment, may output a synchronized sine signal to a comparator 22that may convert the sine signal to a square signal. The square signalgenerated, in embodiments of the invention, may be fed to frequencydivider 20 that may generate a clock for switching the at least oneswitch 16. The clock generated, in one or more embodiments, may includea frequency corresponding to a fraction of the frequency of theoscillatory electric field wherein the fraction may be determined by afrequency division factor applied by frequency divider 20.

Alternatively, in at least one embodiment, a control device to controlthe at least one switch 16 of the implantable medical device 10 may beconnected to a pulse modulator which may not be synchronized to theexternal electric field. For sufficiently low switching frequenciesrelative to the external field oscillatory frequency, for example 1:100frequency ratio, in at least one embodiment, the impact of lack ofexternal field phase synchrony to the impedance modulation caused byswitching may be low. Only 1% sensitivity reduction of the externaldevices ability to detect the impedance changes induced by theimplantable medical device would be caused by lack of phase synchronyfor this example ratio.

The actual switching of the at least one switch 16 may further depend ondata that may be transmitted from the implantable medical device 10 tothe external device 12, according to embodiments of the invention. Thisdata to be transmitted may be coded and the code may determine theactual sequence of switching of the at least one switch 16.

In one or more embodiments, frequency divider 20 may be a flip-flopcounter. In embodiments of the invention, the at least one switch 16 mayhave a small on-resistance (preferably less than the volume impedance oftissue in which the device is implanted), which may lead to change ofimpedance of the tissue volume containing the implanted medical device,depending on whether the at least one switch 16 is opened or closed.

In at least one embodiment, such change of impedance may be sensed byexternal device 12.

In one or more embodiments, data transmission from the implantablemedical device to the external device may be summarized as follows:

Apply signal (oscillatory electric field)→propagate in body→switchon/off of the at least one switch 16 in implantable device→impedancechange of body→detect change by external device.

By way of one or more embodiments, during phase-synchronization, theswitch control of the implantable medical device 10 may receive an inputsine signal by detecting a voltage across electrodes 18 or across aresistor. The switch control of the implantable medical device 10according to at least one embodiment of the invention, as shown in FIG.3, may include a band pass filter 26, phase-locked loop 24 that may lockin the frequency of the input sine signal, comparator 22 that mayconvert the sine signal to a square signal, a flip-flop counter that mayact as frequency divider 20 that control the at least one switch 16. Inembodiments of the invention, the at least one switch 16 may include asmall on-resistance.

In at least one embodiment, during free-running switch control, theswitch control of the implantable medical device 10 may control the atleast one switch 16 opening and closing without regard to the phase ofthe external electrical field. The at least one switch 16, in at leastone embodiment, may have a small on-resistance relative to the tissuevolume impedance in which it is implanted.

FIG. 4 shows a more detailed representation of an external deviceaccording to one or more embodiments of the invention. In at least oneembodiment, the impedance changes caused by the implantable medicaldevice 10 may be detected by the external device 12. As shown in FIG. 4,the external device 12 may include a lock-in amplifier that may generatean output signal (signal c) that may represent the signal transmitted byimplantable medical device 10 by way of impedance changes. Lock-inamplifier 30, in one or more embodiments, may use the signal imposed ona body using cutaneous electrodes 14 as a reference signal. For thispurpose, a network of resistors 32 may be provided that may cause avoltage drop representing the signal (the oscillatory electric field)imposed on a body via cutaneous electrodes 14.

In at least one embodiment, this signal may be amplified bypre-amplifier 34 of lock-in amplifier 30.

By way of one or more embodiments, the amplified signal sensed viacutaneous electrodes 14 and the resistor network 32 may be fed to an AMdemodulator that may include a phase-sensitive detector 36 and furtherfed to a low pass filter 38, as depicted in FIG. 4. The amplified inputsignal sensed via cutaneous electrodes 14 and the resistor network 32 isrepresented as signal (a) in FIG. 4. The output signal of thephase-sensitive detector 36 is depicted in FIG. 4 as signal (b). The lowpass filtered output signal of lock-in amplifier 30 is depicted in FIG.4 as signal (c). Signal (c), in at least one embodiment, may correspondto the signal generated by implantable medical device 10 and thus mayrepresent data to be transmitted from implantable medical device 10 toexternal device 12. In embodiments of the invention, this signal may beanalog-to-digital converted and stored. Block 40 in FIG. 4 represents ananalog-to-digital converter (ADC), a memory for data storage and adisplay of external device 12.

For the detection of impedance changes of body caused by the implantablemedical device 10 the external device 12, in at least one embodiment,may include a lock-in amplifier that may use the input as referencesignal, comprising an AM demodulator, which in turn may include aprecision rectifier and a low-pass filter. In embodiments of theinvention, the low pass filtered signal may be fed to ananalog-to-digital converter or level discriminator in order to digitizethe received signal. Due to the sensitivity of the phase lockeddemodulator (lock-in amplifier), the impedance changes detected may bevery small, on the order of −120 dB.

The communication from implantable medical device 10 to external device12 may be understood as follows.

As the implantable medical device 10 alternatively shorts and opens theconnection between the electrodes 18, in one or more embodiments, theimpedance between the external device electrodes 14 may be slightlychanged or modulated. The external device 12 may sense the change inimpedance by measuring how much voltage or current is imparted on theelectrodes 14 to create the oscillator electric field between theexternal device electrodes 14.

In at least one embodiment, the external device 12 may sense the changein impedance using a lock-in amplifier that may be synchronized to theelectric field frequency and phase. As the implantable medical device 10modulates the impedance between the external device electrodes 14, in atleast one embodiment, the external device 12 may integrate the changingcurrent or voltage. The integration may allow a very small change insourced voltage or current to be detected using amplitude modulation.

In one or more embodiments, communication may occur at approximately1/10 to 1/1000th of the modulation frequency of the oscillatory electricfield. This allows for ˜10-1000 cycles of the electric field to beintegrated to determine the imparted current or voltage. A model andsimulation of the passive z-com embodiment is shown in FIGS. 5 and 6 aswill be discussed below.

In at least one embodiment, a small electric field may be generated bythe implanted device in place of passive resistance changes. In moredetail, an external device 12 may generate a first oscillating electricfield using cutaneous electrodes 14. In one or more embodiments, theimplanted device 10 may detect the first oscillating electric field byelectrodes 18 and may acquire a phase lock to the first oscillatingelectric field using a phase locked loop. The externally induced firstoscillatory electric field may briefly be turned off by the externaldevice, while the phase locked loop maintains its phase information. Inembodiments of the invention, the implanted device may then generate asecond oscillatory electric field using electrodes, the field having afixed phase relationship with the first oscillatory electric field. Theexternal device 12 may detect and demodulate the second oscillatoryelectric field by cutaneous electrode 14 and a phase-locked demodulator.Changing amplitude, phase, or other common modulation techniques, inembodiments of the invention, may be used by the implanted medicaldevice to modulate the second oscillatory electric field, thus includinga communications device to transmit data to the external device 12.

In at least one embodiment, the implanted device may switch itsimpedance or small electric field at every other crest of the externaldevice's applied electric field for ˜10-1000 cycles, called a chirp, inorder to represent a mark of the communications scheme.

In one or more embodiments, a communication from external device 12 toimplantable medical device 10 may be done as follows:

The imposed oscillatory electric field may include a fundamentalfrequency that the implantable medical device 10 uses to lock onto. Thefundamental frequency may also be used as a carrier frequency to sendmodulated data to the implantable medical device 10. The external device12 may modulate data, in at least one embodiment, using frequencymodulation or amplitude modulation, on top of the carrier impartedelectric field. In embodiments of the invention, the implantable medicaldevice 10 may decode the modulated data sensed through the electrodes.

Embodiments of the invention may not require the implanted medicaldevice 10 to actively transmit data using its own power in the case ofimpedance modulation embodiment. In embodiments of the invention,implanted medical device 10 may modulate a field imparted on it by anexternal device 12. In embodiments of the invention where the device mayemit a low amplitude electric field, the phase-locked property that itmaintains may allow for lower amplitude to be used for transmission tothe external device 12 than if simple band-limited transmission schemeswere used. In embodiments of the invention, the resulting drain on themedical device's battery may be extremely small. Because of the smallpower consumption, it may be possible to transmit more data to theexternal device 12.

FIGS. 5 and 6 illustrate a simulation of passive impedance-based datacommunication. Specifically, FIG. 5 shows a circuit diagram for asimulation scenario for simulating impedance change based datacommunication according to one or more embodiments of the invention, andFIG. 6 shows a plot illustrating a simulation result achieved by usingthe diagram depicted in FIG. 5 according to one or more embodiments ofthe invention.

Standard electrode-electrolyte-body impedance models and blood impedancemodels, according to one or more embodiments, may be combined with alock-in demodulator in a model of the passive impedance implementationof the invention. Each simulation component is labeled, with the‘device’ modeled in box labeled with “10” with access impedance and fourclosing switches to simulate the communication of two data bits to theexternal receiver/demodulator. In at least one embodiment, environmentaland breathing noise may be induced in the model with voltage sources V3,V4, and V5 simulating line noise, RF noise, and breathing, body changingimpedance noise, respectively.

The simulation signal at output filter resistor R21, in at least oneembodiment, is shown as the green trace in FIG. 6. The impedancechanges, in embodiments of the invention, may be at a detectable level(showing 2 bits) beyond the background noise that has been filtered outdue to the lock-in receiver system. The slope to the left of this plotshows the final settling time of the filters.

It will be apparent to those skilled in the art that numerousmodifications and variations of the described examples and embodimentsare possible in light of the above teaching. The disclosed examples andembodiments are presented for purposes of illustration only. Therefore,it is the intent to cover all such modifications and alternateembodiments as may come within the true scope of this invention.

What is claimed is:
 1. A method of communicating data from animplantable device to an external device, the method comprising:imposing an oscillatory electric field on body tissue surrounding animplantable device, sensing the oscillatory electric field imposed onthe body tissue via the implantable device, one or more of altering theoscillatory electric field via the implantable device, by inducing analternating change of impedance via the implantable device or generatinga small oscillatory electric field via the implantable device, andgenerating a second oscillatory electric field which is substantiallyphase synchronized to the oscillatory electric field imposed on the bodytissue via the implantable device, and sensing the change of impedanceor the small oscillatory electric field generated by the implantabledevice, respectively, using an external device, wherein the externaldevice comprises or is connected to at least two cutaneous electrodes.2. The method according to claim 1, wherein the altering of theoscillatory electric field via the implantable device is performed by atleast two electrodes operatively connected to or part of the implantabledevice, and by at least one switch operatively connected to the at leasttwo electrodes, wherein the at least one switch is configured to connectand disconnect the at least two electrodes in an alternating manneraccording to a code generated by the implantable device representingdata to be transmitted from the implantable device to the externaldevice, and wherein said alternating connecting and disconnecting of theat least two electrodes causes a detectable change of impedance of theimposed oscillatory electric field.
 3. The method according to claim 1,wherein the sensing of the change of impedance comprises sensing acurrent or voltage over the at least two cutaneous electrodes andintegrating a sensed voltage or current over a number of cycles of theoscillatory electric fled, wherein said number of cycles correspond to afrequency division factor of a frequency divider of the implantabledevice.
 4. The method according to claim 1, further comprisingcommunicating data from the external device to the implantable device,and further comprising modulating the oscillatory electric field usingthe external device.
 5. The method according to claim 4, wherein theoscillatory electric field imposed on the body comprises a fundamentalfrequency that the implantable device uses to lock onto.
 6. The methodaccording to claim 5, wherein the fundamental frequency is used as acarrier frequency to send modulated data from the external device to theimplantable device.
 7. The method according to claim 1, wherein thesensing of the change of impedance comprises sensing a current orvoltage using the external device, and further comprising demodulatingthe change of impedance sensed to generate a communication scheme totransmit data from the implantable device to the external device.
 8. Themethod according to claim 2, wherein the code comprises a clock, andwherein the clock is a fraction of a frequency of the oscillatoryelectric field imposed on the body tissue surrounding the implantabledevice.
 9. The method according to claim 2, wherein said detectablechange of impedance of the imposed oscillatory electric field isdetected by the external device by measuring an amount of voltage orcurrent imparted on the at least two cutaneous electrodes.
 10. Themethod according to claim 2, wherein the at least one switch isconnected to a switch control, and further comprising synchronizing theconnecting and disconnecting of the at least two electrodes with theoscillatory electric field imposed on the body tissue via the switchcontrol.